Method of cleaning hot-dip tin-plate



July l, 1952 P. N. MURPHY METHOD OF CLEANING HOT-DIP TIN-PLATE FiledApril e, 1948 avoided.

ent .manufacturing practice.

Patented July 1, 1952 UNITED N STATE METHOD oFCLEANING.Hor-DirfriN-PLATE 4Paul N; Murphy, Hobart, Ind., assignortoUnited States Steel Company, a corporation lof New Jersey Appucautnnpril6, 1948, serialize. 19,30

. 1 'This 'invention relates .tothe manufacture of tin-plate and, inparticular, to animproved finishingtreatment thereof, i. e., theprocessing subsequent to the coating with tin, whereby discoloration orthe Ikformation of surface film is Prime tin-plate should exhibit abrightsilvery appearanceandbe free fromwisible surface iilms ordiscoloration.'but completefreedoxn from vdiscoloration is .diiioult .toachieve under "the 'pres- Two types of discoloration are frequentlyencountered." The first is .the development vof a straw-yellow color onthe sheet "when it lis heated incident `to lithographing orotherprocesses duringfabrication. The second 'is the appearance of asimilar or even deeper color on the sheets before shipment fromthemanufacturer'tothe customer. It has been observed that the higherthetemperature of the palm-'oil `bath through which theytinned sheetsemerge from the moltenl tin in the ordinary hot-dip tinning process,vthe greaterthe tendency to discoloration. `If -the palm `oil is keptbelow the temperature which causes discoloration, however, theresultisthe formation 'of heavy list edges and beadsof tin on thesheets.

While such defects werenot considered very se- .riousinthe past, theyhavenow become so be- 'ca'use vof the increasingly high standards of"ing, however, because it rhas been generally accepted that a solution"strong `enough to vdo so would etch the sheets. `As a result, theamount of oil remaining cannot be precisely controlled and is'frequently more or less than is `considered `desirable by the user.rI'he branning operation is objectionable,furthermorabecause of the dustcreatedthereby `in the tin house and further because the branucontaminates the 'surface of the material and results in eye-holing"`or discontinuousadherence of the .lacquer film applied 'tothe ,sheetsbefore .fabrication `fof certain arlticles, such as .packing cans. .Many`sheets are cobbled .or otherwise Qdanaged in' the branning' machine,furthermore, `andha've to be scrapped.

It is accordingly the primary object jof :my

. invention Ato. provide la method of 'n'ishing tin- `plate whereby theproduct is free from'discoloration as well as Yheavy list edges andbeads.

A further object `is `to simplify the -nishing process and reduce thecost thereof. A `still further object is Vto eliminate the branningoperation entirely. Another object -is -to Iachieve complete removal ofoil from the sheets during the washing.

In a preferred practice of the invention, VI deliver sheets of hot-dip`tin-plate, astheyemerge all palm oil from the sheets.

zov

`rinsed with hot water.

from the palm oil bath -oating on the molten tin in the tin-pot, withoutcooling and while `the `oil adhering thereto is still fluid, `directlylinto a wash tank. After immersion in a solution of soda ash containedtherein, they are fconveyed to a second wash tank containing a similarbut less concentrated solution and are lthen -spray- The first 1washing.solution is strong enough to remove substantially As a .nnaloperation,` therefore, I apply to the sheets .just the `precise amountof oil desired by the trade. A complete understanding of the inventionmay be obtained from the following detailed description and explanationwhich refer to the Vaccompanying drawings illustrating apparatus forcarrying out 'the presentrpreferred practice. the drawings:

Figures 1V and i2 together constitute a -central vertical sectionthrough a :line of apparatus for subjecting tinned sheets `to the`finishing i is of Yknown construction andthe showingithereof 'is'largely diagrammatic since no detailed description is -necessary. It hasaplurality 'of fexit rolls I'6 therein. Deflecting yrolls |612 above the4tin-pot progressively bend .the sheets emerging successively therefromfor .delivery laterally of the pot. A guide chute I1 extends downwardlyfrom 1a point adjacent the delector rolls and directs each sheet betweena pair of entry or yfeed-'in rolls I8 journaled `on the near rsid'e ofvthe wash tank I I.

Additional .pairs of traction lrolls I9 and 2B convey the sheet throughIthe "washing'solutioncontained in the tank Ill. The

4sheet is then passed between two Apa-irs of iwiping rol-ls 2'I whichremove excess `washing .fso-

L lution by squeegee action, and discharged fonto InY y I1.vcomplete/removal' of the oil from the sheet in any suitable means.

which it enters the second wash tank I2.

accises tional traction rolls 24 and 25. The last of these delivers thesheet to two pairs of wiping rolls 26 which remove excess solutiontherefrom and deliver the sheet to a roller table 27. As the sheetpasses over the table, it is subjected to a hotwater rinse by spray jetsfrom pipes I3. A tank 28 is provided below the table for collecting therinse Water flowing from the sheet.

The oiler I4 through which the sheet then passes comprises a pair ofpinch rolls 29 which feed the sheet forward onto a roller table 3|).vv

Oil is supplied to the rolls 29 by a stack of rolls 3|, the upper one ofwhich engages the bottom pinch roll. The bottom roll 3| rides on a wick32 immersed in palm oil contained in a reservoir 33. The rolls 3| areprovided with wiper blades 34 for controlling the amountof oiltransferredY thereby.

After passing through the oiler and being delivered by the pinch rollsthereof onto the table 30, the sheet is engaged by pinch rolls I and`delivered thereby to a piler (not shown).

-The apparatus preferred for carrying out my g improved method havingbeen descrbed'in detail, it remains to explain further the successiveoperations performed thereby upon the Vtinned sheet. The operation ofthe tin-pot is similar to prior practice except that I maintain thetemperature of the palm-oil bath floating on the molten tin at atemperature between 460 and 480 F. The upper limit is controlled so thatthe -tin coating will be solidified by the time the sheet reaches thelast set of exit rolls I6. This facilitateshandling of the materialduring subsequent processing. Moreover,

With the oil at the temperature stated, it remains highly fluid as thesheet bends over and falls down onto the guide This is advantageous fromthe standpoint of the two-stage cleaning afforded by the tanks II andI2. To insure entry of the sheets into the first wash tank beforesubstantial cooling of the sheet and stiifening of the oil thereon, therst wash tank is placed as close as possible to the tin-pot and thecooling conveyor, usually incorporatedin a tinning line, is omitted ormade as short as possible.

The first wash tank II contains an aqueous solution of from 0.5% to`l.5%of soda ash or anhydrous sodium carbonate.

The solution is heated toa temperature of from 180 to 200 by Theconveying rolls in the wash tank are driven and preferably the two rollsof each` pairare driven at slightly different speeds, thereby producinga rubbing action on the sheet. l The speed of the rolls is such as tocause the sheet to be immersed in the solution for -from Y5 to l0seconds.

` The solution in the first wash tank, by`reason of its concentrationand temperature, is effective to remove most of the I oil adhering tothe sheet as a result of its passage f through the palm-oil bathfloating on the molten f tin, on emerging from the tin-pot. rolls 2|remove washing solution adhering to the The wiping sheet and deliver ittothe roller table 22 from This tank contains an aqueous solution offrom 0.10% to 0.25% of soda ash or anhydrous sodium carbonate and ismaintained at approximately the same temperature as the solution in therst wash tank. The rolls in the tank I2 are driven in the same manner asthose of tank I Ii. As the it obviously eliminates v heavy list edgesand beads of tin which would adhere to the sheet if the oil weremaintained at a lower temperature.

, frequently occurs.

oil on both sides thereof.

`transferred from the wick to the pinch rolls 29 sheet passes throughthe solution in tank I I, the last trace of adherent palm oil isremoved. Excess washing solution is removed from the sheet as ittraverses the rolls 36 and it is then thoroughly rinsed by the sprayjets of hot water (-200 F.) from pipes I3 as it passes along the table21.

The nal operation of my improved method is the application to the sheetofv a'predetermined amount of palm oil as it passes through the rolls 29of oiler I4. It will be evident that, in the interval between thepassage of successive sheets, the rolls 3| will transfer a film of oilfrom the wick to the rolls 29. This lm is applied to the sheet as itpasses between the last-mentioned rolls. As a result, the sheetsemerging onto the roller table 30 have a predetermined coating of Theamount of oil and applied thereby to the sheet is controlled byadjusting the Wiper blades 34 engaging the transfer roll 3| and varyingthe speed of the latter. The finished sheets, coated with palm oil, aredelivered by the lpinch rolls I5 to a piler and, after piling, are readyto be stored or shipped.

It will be apparent from the foregoing that the method of my inventionis characterized by numerous advantages. In the first place, the sheetsproduced thereby are characterized by unusual freedom from discolorationdespite the fact that the palm-oil bath in the tin-pot is maintained ata higher temperature than has heretofore been thought feasible. As amatter of fact, the material produced by my method is entirely free fromdiscoloration when shipped'and does not develop this defect whenlithographed under normal conditions. The tendency of tin-plate todiscolor is graded on a numerical scale from zero to ten, the latterindicating very bad discoloration. vMy method makes possible theproduction of tin-plate possessing stain resistance of grades 1 and 2.This compares with the production of grades 3 and 4 by the conventionalmethod of finishing tin-plate.

A further advantage is that the luster and reflectivity of tin-plateprocessed by my method are greater than that of the product resultingfrom the prior practice while no spangling is produced. My method,furthermore, eliminates the branning treatment altogether and thus doesaway with difficulties resulting from eye-holding as well as theobjectionable contamination of the air in the vicinity of the processingapparatus by bran dust. Finally, the percentage of prime sheets producedis substantially increased as a result of the elimination of the brannerin which cobbling'or other damager to the sheets As a result, theinvention makes possible a reduction in maintenance and productioncosts.

Although I have illustrated and described but a preferred practice ofthe invention, it will be understood that changes in the proceduredisclosed may be made without departing from the spirit of the inventionor the scope ofthe appended claims. Y

I claim: Y Y

1. In a method of finishing hot-dip tin-plate, the steps includingdelivering sheets as they emerge from the palm oil oating on the moltentin in a tin-pot, before substantial cooling of the sheets, directlyintoa hot soda-ash solution having a concentration between .5% and 1.5%,then passing the sheets through a second hot soda-ash solution having. aconcentration between .10%

6. REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date Ferm Dec. 27, 1938 Rath Sept. 12,1944 Croft Sept. 26, 1944

1. IN A METHOD OF FINISHING HOT-DIP TIN-PLATE, THE STEPS INCLUDINGDELIVERING SHEETS AS THEY EMERGE FROM THE PALM OIL FLOATING ON THEMOLTEN TIN IN A TIN-POT, BEFORE SUBSTANTIAL COOLING OF THE SHEETS,DIRECTLY INTO A HOT SODA-ASH SOLUTION HAVING A CONCENTRATION BETWEEN .5%AND 1.5% THEN PASSING THE SHEETS THROUGH A SECOND HOT SODA-ASH SOLUTIONHAVING A CONCENTRATION BETWEEN .10% AND .25%, THEREBY REMOVINGSUBSTANTIALLY ALL THE OIL ADHERING TO THE SHEETS ON LEAVING THE TIN-POT.